In recent years, a plating method known as “through-hole plating” or “via filling plating” has been used in basic production of printed circuit boards used in electronic devices such as personal computers. Electrolytic copper plating is expected to have application to through-hole and via plating, since the rate of deposition of the plating film is rapid, 10-50 μm/hr. However, if the copper is deposited on the whole inner via surfaces, the rate of deposition near the bottom of the vias must be more rapid than the rate of deposition in their opening parts in order for the inside of the vias to be filled with copper without leaving voids. If the rate of deposition near the bottoms is the same as or slower than the rate of deposition in the opening parts, either the vias will not be filled, or the opening parts will plug up before the copper plating filling inside the vias is completed, voids will be left inside them, and in either case they will not be suitable for use. Moreover, in through-hole plating, there is a requirement that the covering ability of the plating in the through holes, known as “throwing power,” be good.
Up to now, electrolytic copper plating baths containing specific compounds which contain sulfur atoms have been used to accelerate the deposition rates near the bottoms of vias and on the walls of through-holes; as for the electrolysis conditions, direct-current electrolysis using soluble anodes, such as phosphorus-containing copper anodes, has generally been used. With this method, however, although there is a good via filling performance immediately after the bath is made, the electrolytic copper plating bath becomes unstable over time and problems are produced after a certain period of time has elapsed after the formation of the bath, including the facts that particle lumps are produced in the formation of the electrolytic copper plating layer, the external appearance of the plating becomes worse, the filling of the via becomes unstable, etc. Furthermore, the reliability of the resistance to thermal shock and the throwing power are reduced in through-hole plating.
In order to solve these problems, an electrolytic copper plating solution which contains specific compounds containing sulfur atoms and thiol reactive compounds is disclosed in Japanese Unexamined Patent Application No. 2002-249891. As the thiol reactive compounds, aliphatic and alicyclic compounds, carboxylic acids, peroxo acids of aromatic or heterocyclic compounds, aldehydes and ketones, and hydrogen peroxide are disclosed, and it is stated in the working examples that formaldehyde improves the filling ability. In recent years, however, considering the effects of formaldehyde on the environment and the human body, the fact that its flash point is low (66° C.), etc., efforts have been made to find outer compounds with via filling ability improving performances to substitute for formaldehyde. Moreover, although an electrolytic copper plating solution is disclosed in Japanese Unexamined Patent Application No. 2011-207878 which contains accelerants (glossifying agents) containing sulfur atoms and reaction products of nitrogen-containing compounds and epoxide compounds, there is no mention in this reference to the problem of degradation products due to changes over time in the accelerants containing sulfur atoms.